1. Technical Field
The present invention relates to printed circuit boards used in large computing systems, and in particular to a motherboard assembly having stacked land grid array (LGA) connectors for interconnecting and distributing signals and power therein.
2. Description of the Related Art
The electronics industry widely uses electrical connectors. In many computers and other electronic circuit structures, an electronic module or chip, such as a central processor unit (CPU), memory module, application-specific integrated circuit, or other integrated circuits having a module substrate, connects to a printed wiring board using an electronic connector. Printed wiring boards are also known as printed circuit boards (PCBs) and etched wiring boards. A printed circuit board is commonly referred to as a printed circuit board assembly (PCBA) when it is populated with one or more electronic modules. When a PCBA is used as the central PCB in a complex electronic system, it is commonly referred to as a motherboard. To connect an electronic module to a PCB or motherboard, a plurality of individual electrical contacts on the base of the electronic module must connect to a plurality of corresponding individual electrical contacts on the PCB/motherboard.
When an LGA connector connects the electronic module to the PCB, this set of contacts is known as land grid arrays (LGAs). A LGA is a type of surface-mount packaging where there are no pins on the module. Rather, in place of pins are pads of gold-plated copper, for example, that couple to pads on the PCB (e.g., motherboard). Rather than permanently soldering the electronic module contacts to the LGA site, it is desirable to use LGA connectors that allow the user to install and remove the electronic module to/from the LGA site. LGA connectors provide the user with the flexibility to upgrade or replace electronic modules during the manufacturing cycle and in the field. LGA connectors are also known in the art as sockets, interconnects, interposers, carriers, and button board assemblies. In general, LGA connectors provide electrical connections between two parallel electrical substrates in computing equipment through the use of an interposer. Typically, one of these substrates is a PCB (e.g., motherboard) and the other is an electronic module, which may, for example, have either a ceramic or organic laminate substrate.
PCBs typically include multiple conductive layers laminated with insulating plastic there between. The conductive layers are typically reserved for power, power return, ground, and signals. The layers reserved for signals contain etchings to form “traces” that conduct the signals. The layers reserved for power, power return and ground are typically referred to as “power planes”, “power return planes”, and “ground planes”. The conductive layers connect together by drilling holes called vias and then plating each via with a conductor to form a plated-through-hole (PTH).
In large symmetric multiprocessing (SMP) computer systems, it is advantageous to package as much of the system as possible within a single rack drawer. To maximize component density as a function of printed circuit board density, it is desirable to package an entire system on a single printed circuit motherboard. FIG. 1 is a top perspective view of motherboard 100 having many modules, including high power modules 102, such as CPUs, associated memory modules 104, as well as “concentrator” modules 108 such as hubs 1-8. Unfortunately, the ability to manufacture a large enough motherboard that is capable of placing all such modules shown on motherboard 100 is limited by the existing standard tooling available by printed circuit board manufacturers. Generally, the larger the PCB, the smaller the yield because there are more risk sits on the PCB. At some point, the cost to manufacture a PBC of a certain size or larger becomes prohibitive. For example, a vendor with large panel capability can manufacture PCBs with active areas up to a dimension of 30.35″ by 22.87″, but no larger. Because motherboard 100 requires a dimension larger than this, PBC vendors are not tooled to manufacture motherboard 100. It is simply too large.
Alternatively, to generate an assembly having a large enough surface area, but which can be manufactured using standing tooling, motherboard 100 may be divided into two coplanar printed circuit boards and connected using a connector, such as a right angle-to-right angle or coplanar connector. Coplanar PBCs mean two or more boards lying in the same plane. FIG. 2 is a top view perspective of an illustrative conventional PCBA 200 having high power modules 202 positioned on PCB 204, and concentrator modules 208 positioned on PCB 210. Right angle interconnect 206 connects coplanar PCBs 204 and 210 such that they function as a single motherboard. Because PCBs 204 and 210 are each much smaller in terms of X,Y dimension than motherboard 100, they may be manufactured using standard large panel tooling. Unfortunately, however, the system drawer (not shown) housing PCBA 200 must grow to accommodate the width of right angle interconnect 206 as an addition to the card surface area. Moreover, the use of interconnect 206 may degrade signal integrity because signals traveling between PCBs 204 and 206 must pass through it.
It should therefore be apparent that a need exists for an enhanced mechanism for interconnecting and distributing signals and power between coplanar boards in large systems that consume minimal additional surface area.